The present invention relates to a method of focusing a charged particle beam and to a plasma lens for focusing a charged particle beam in accordance with the method, preferably for the collection of divergent rays for the transmission and focusing of charged particle beams which are generated in an accelerator and which are focused by means of a magnetic field produced by current flow through a plasma.
The present invention is generally concerned with a space-charge compensating transmission and the fine-focusing of high-flux charged particle beams by means of a magnetic lens which has a linear mapping characteristic of the first order in both planes transverse to the beam at the same time. For beam transmission and focusing of charged particle beams, electric and magnetic quadrupoles have been used exclusively so far. Quadrupoles however have the disadvantage that they focus only in one plane while, at the same time, they de-focus in the other plane. Focusing of a beam in both planes can be achieved with this type of lens only by a serial arrangement of several quadrupoles (multiplets). Furthermore, the focusing strength of quadrupoles is limited by the field strength achievable at the pole piece.
Magnetic horns and wire lenses are special lenses which provide for strong focusing in both planes. The magnetic horn provides for focusing in both planes at the same time but it has no linear characteristics of representation. This results in partial absorption and scattering of the beam to be focused. It is used therefore almost exclusively for the collection of divergent rays.
Plasma lenses are a special type of wire lens wherein the conductive material, that is, the "wire", is represented by highly conductive plasma through which a high current is pulsed. The practical realization of such lenses however is very involved and expensive. The large axial current pulses cause, during discharge, a constriction of the plasma in a direction normal to the z-axis (the longitudinal axis of the beam), the so-called z-pinch, which, for a plasma lens, should coincide with the maximum current flow. At the same time, various instability modes are rapidly developing which limit the duration of the pinch and accordingly the focusing effects. It is further noted that although the known plasma lenses are capable of generating high magnetic fields, they achieve only limited accuracy of field linearity in the plasma.
A plasma lens of the type described herein is known from "PHYSICAL REVIEW LETTERS", Vol. 66, No. 13, p. 1705 ff. The plasma lens described in this article is based on the z-pinch discharge phenomenon and requires, for the ionization of the gases for the acceleration of the plasma, a relatively high amount of primarily stored energy which counteracts a fast pulse operation. Furthermore the location of the pinch axis as well as the timing of the pinch are subject to a certain lack of focus (jitter) because of rapid dynamic changes. With a z-pinch discharge the largest amount of energy stored in the condenser arrangement is utilized to compensate for the gas or the plasma. With the arrangement referred to above the needed amount of stored energy was typically 7-10 KJ. This enormous amount of energy substantially limits the repeat frequency of the discharge to about 1/minute. However, for good focusing only the magnetic properties are important, the density of the plasma is of minor importance in this connection. The life of the pinches is, as already mentioned, limited by the plasma instabilities to no more than several 100 ns.
On the basis of the state of the art it is the object of the present invention to provide a novel focusing method which is independent of the z-pinch and to provide on the basis of the novel method a novel plasma lens which does not have the disadvantages pointed out earlier. In particular the current should flow through the whole discharge cross-section as homogeneously as possible without causing dynamic constriction of the plasma.